My main goal in this project was to develop a system for studying the genetic basis for mechanisms by which Lactobacillus strains colonize the keratinized murine gastric epithelium. Lactobacillus strains from various animal sources were tested for their capacity to colonize that habitat. Only strains isolated from rodents could colonize the gastric surface. The strains were then screened for plasmids. Of 24 tested, twenty strains contained cryotic plasmids that could not be cured by methods used. There was no obvious relationship between the plasmids detected and 3 phenotypes of the bacterial strains (i.e. resistance to antibiotics, deconjugation or bile acids, and colonial morphology). Plasmids of 80-90 Mdal were detected only in strains isolated from mice and able to form layers on the gastric epithelium. DNA-DNA hybridization with one of those large plasmids as probe, and analysis with five restriction endonucleases indicated that these large plasmids belong to at least two groups. DNA sequences homologous to the probing plasmid were detected in six of the strains studied. All of the six contained plasmids of various sizes. Only some of them colonized the keratinized gastric epithelium in monoassociated mice. No apparent relationship exists, however, between any of the plasmids detected and the capacity of the lactobacillus strains to colonize their gastric habitat.To facilitate the genetic study of mechanisms mediating the colonization, a transformation system was developed for L. acidophilus 100-33, a straon isolated from pig feces that could not colonize the murine gastric epithelium. Strain 100-33 wild-type was transformed to rifampicin resistance with DNA derived from spontaneous mutants resistant to rifampicin derived from that strain. Transformation frequency was up to 5.7 x 10('-7) transformants per regenerated cell per (mu)g DNA. The genetic characte stably maintained in the transformed cells.Cells of strain 100-33 wild-type demonstrated to be competent in an experiment of the type just described above were transformed to be able to colonize the murine gastric epithelium with a preparation containing both the chromosomal and plasmid DNAs from a Lactobacillus strain harboring the probing plasmid used in the hybridization experiments. The purified DNA of the probing plasmid itself failed to transform strain 100-33 to that capacity. Therefore, the mechanisms by which lactobacilli colonize their gastric habitat are probably coded by genes on the bacterial chromosome.